The development of sustainable, i.e. cost-effective and environmentally friendly, highly-selective processes for the production of fine chemicals (pharmaceuticals, agrochemicals, fragrances, etc.) is a current major concern at the industrial level. At present, most industrial processes showing high activity and selectivity, particularly stereo- or enantio-selectivity, are based on the use of homogeneous-phase, molecular catalysts. These compounds commonly consist of heavy (noble) metal complexes containing highly elaborated (chiral) ligands. Besides being complicated to be prepared and expensive, these catalysts suffer from the difficulty of their recovery from the reaction mixture and their reuse. Also, separation of the products from the catalyst and the solution (usually an organic solvent) invariably leads to the emission of volatile pollutants.
In order to solve these problems, the inventors suggested new type of catalytic materials, in which the metal complex molecular catalysts are immobilized in an inorganic/polymeric hybrid material support (PCT/JP2010/056288WO 2011/121797). The inorganic/polymeric hybrid materials are the hybrids of inorganic oxide, such as silicic acid compound and tungstic acid compound, and organic polymers, mainly polyvinyl alcohol (PVA), combining chemically each other. These hybrid materials are produced by simple processes in aqueous solution, in which salts of inorganic oxides are neutralized by acid with PVA co-existing. In this method, the nascent and active inorganic oxides generated by neutralization combine and hybridize with PVA to form the hybrid compounds. The hybrid compounds are distinguished from mixtures of inorganic oxides and PVA, that is, their chemical properties are remarkably changed from their raw materials. For example, once hybridized materials are insoluble in any solvents including hot water. In addition to that, these hybrid materials have extraordinarily high thermal and chemical stability.
The preformed metal complex catalysts able to induce stereo- or enantio-selectivity in hydrogenation reactions of organic substrates, such as [(−)-BINAP)Rh(NBD)]PF6 and [(−)-(Monophos)2Rh(NBD)]PF6, can easily and stably be immobilized not only on the surface but also inside the preformed inorganic/polymeric hybrid materials. The stereo-selectivity of the reaction is almost kept even when the metal complex molecular catalysts are immobilized into the hybrid materials. These catalytic materials, which are heterogeneous catalyst, can easily be recovered from the reaction mixture, and reused. Furthermore, these catalytic materials absorb solvents, especially the high polarity solvents like as methanol, to swell and also absorb reaction substrates, so that chemical reactions proceed not only on the surface but also inside the catalytic materials. It contributes to both the higher activity and the lower metal leaching into solution.
On the other hand, the inorganic/polymeric hybrid materials were applied to another kind of catalytic material by the inventors (PCT/JP2011/065129). In this kind of catalytic material, metal nano-particle (MNP) catalysts are embedded in the inorganic/polymeric hybrid materials.
MNPs, especially those of noble metals, such as platinum, palladium, ruthenium, rhodium and gold, are widely used as effective catalysts in various kinds of chemical processes. In many cases, the MNPs are immobilized onto solid support materials based on porous inorganic materials, such as carbon, silica, titania or alumina. A common strategy to immobilize the MNPs onto a support material is the impregnation method in which the support is immersed into a solution of a metal precursor, dried and calcined. After that, the metal is reduced by some reducing agent to form MNPs.
However, it is difficult to control the particle size by this method, as the size distribution can be wide with particles beyond ten nanometers or more. In addition, the catalytic materials of this type are often used in the form of fine powders, so it is not easy to separate the catalysts from the reaction solution. Very fine powders may also clog or poison the reactors or the autoclaves employed in the chemical reaction. Even when the catalytic materials are not fine powders, the support materials may also pulverize upon agitating. Furthermore, the MNPs on the support materials tend to aggregate upon use to form larger particles having smaller surface area and, hence, lower activity, ultimately resulting in catalyst deactivation after prolonged use. Metal leaching from the catalyst to the reaction solution may also represent a serious problem in terms of contamination of products for the fine chemical (pharmaceutical, perfumery) industry.
Some of the above problems can be solved by using the inorganic/polymeric hybrid materials as support for the MNPs. Due to the absorbency of solvents and reaction substrates, chemical reactions catalyzed by MNPs embedded in the hybrid materials occur both on the surface and inside the hybrid materials, resulting in a high catalytic activity. As the MNPs are embedded in the hybrid materials, aggregation of the MNPs is hampered resulting in a constant catalytic activity upon reuse. Especially, as the MNPs are introduced into the inorganic/polymeric hybrid materials as one of the inorganic constituents, the MNPs can not grow large and stay within nano-size resulting in stably high activity. Embedding the MNPs in the hybrid materials strongly limits their leaching into solution upon use. Although the hybrid materials have the properties of inorganic oxides, they also have flexibility of organic polymers and are not brittle, so the hybrid materials make it possible to avoid pulverization problem.
As mentioned above, the two types of the inorganic/polymeric hybrid catalytic materials, the metal complex molecular catalyst type and the MNP catalyst type, posses some definite advantages comparing to the conventional heterogeneous catalysts. However, they still have some problems, for instance, their catalytic activity strongly depends on the species of solvents. These conventional hybrid catalytic materials are not able to work efficiently until they absorb an enough amount of solvents and reaction substrates. Low solvent uptake causes small swelling and low uptake of reaction substrates, ultimately resulting in low catalytic activity. The inorganic/polymeric hybrid materials prefer the solvents with high polarity, such as water and methanol, because of the hydrophilic property of inorganic oxides. Therefore, the hybrid catalytic materials do not exhibit high activity in the solvents with low polarity. It is expected that higher reaction activity is provided by improvement in affinity to the solvents. Even in the case employing methanol as the solvent, the catalytic activity can be enhanced by increase in the amount of solvent uptake.
In the previous patent application (PCT/JP2011/065129), the inventors disclose that the performance of the hybrid catalytic materials can be tuned by the saponification degree of PVA (polymeric constituent of the inorganic/polymeric hybrid). That is, a low saponification degree (high concentration of acetyl groups) enhances the catalytic activity in low-polar solvents. Although it presents a method for expanding the available solvent species, more effective method is required.